Venture HC should work, no problem. The only limitation might be the 24 mb internal memory. That's still enough for several hundred miles of Mapsource Topo 2008 maps, though. And that unit has the same capacity of the more expensive ones as far as tracks go.
Check out Dave Harris' review of the 705 in the
GPS / Navigation area of bikepacking.net. To me the internal battery limitation has always been a deal killer for bikepacking, but he's using a battery extender with some success.
Still, I think you're better off with the Venture for anything more than a day ride. It's more of a dedicated mapping GPS.
I haven't used Mapmyride for quite a while. I use TopoFusion myself (of course) since you can load multiple GPX files, cut and paste them together and upload a maximum detail track. All these steps to require a bit of a learning curve and I'm sure it's not as easy as plotting a road ride on that site.
Let me know if you have questions...